The inspection of complex components, such as electronic printed circuit boards, is a tedious operation requiring considerable observational skill. Apart from basic element by element checking, various techniques have been developed to accelerate the process. One such technique uses a blink system, in which images of a master component and a component being checked are alternated in a suitable display in quick succession. Any discrepancies between the two images tend to stand out due to an apparent blinking of the area of difference. Careful scrutiny is required and the blinking is very tiring to the operator.
Magnified images of the components have been projected to facilitate comparison, but extreme care in observation is necessary, and the high intensity lighting needed to project clear images causes heating problems. Also, in a projection system, it is difficult to mark a faulty element without upsetting image alignment, or shutting off the apparatus to avoid possible burning of the operator's fingers.